Visual information processing in the primate cerebrum begins in the striate cortex and continues in extrastriate visual areas. One of these areas, the middle temporal area (MT), has a high proportion of neurons that respond to stimuli moving in a given direction within a given range of speeds. We have investigated the contribution of these cells to one aspect of oculomotor behavior that requires stimulus motion: the initiation of slow or pursuit eye movements made to moving targets. We have done this by identifying visual cells in area MT related to one part of the visual field, by requiring the monkey to make pursuit eye movements to targets moving within that part of the visual field, and then by evaluating any deficit in initiation of pursuit by the monkey after removal of this area. Lesions were made with an inhibitory transmitter agonist (muscimol) or a chemical that destroys cells but not fibers of passage (ibotenic acid). Our analysis showed that there was a decrease in the velocity of pursuit eye movements on targets moving within the visual field affected by the injection. This decrease in velocity included that which occurred before monkey acquired the visual target as well as shortly after the visual target was acquired. The deficit was limited to eye movements dependent on a calculation of stimulus motion; saccades to stationary targets within the same area of the visual field were not significantly altered. These experiments show that removing a highly restricted area of the prestriate region of cerebral cortex eliminates the monkey's ability to pursue moving targets in this area. The experiments provide information on the cerebral cortical circuits related to the initiation of pursuit eye movements and also provide one of the first demonstrations that a specific area of prestriate cortex subserves a normal functioning of visual behavior.